Astronomy
Using Ground-Penetrating Radar to Find Subglacial Lakes on Mars
Quick fact
In 2018, scientists using the MARSIS radar on the Mars Express orbiter detected a 20-kilometer-wide lake of liquid water buried 1.5 kilometers beneath the south polar ice cap.
Why this is interesting
There might be hidden lakes of liquid water beneath the Martian ice caps. How can we see them from orbit millions of kilometers away?
Read the full explanation
Understanding Using Ground-Penetrating Radar to Find Subglacial Lakes on Mars
Ground-penetrating radar works by sending radio waves toward the surface from an orbiting spacecraft. These waves travel through the ice, and when they hit a boundary between materials, some of the energy bounces back. By measuring the time it takes for the echo to return, scientists can map the depth of different layers. Liquid water is much more reflective than ice or rock, so a strong reflection at a certain depth indicates a possible lake. It's like using sonar to find fish underwater, but with radio waves instead of sound.
A deeper explanation
The key to this technique is that radar waves can penetrate ice with little loss, but when they encounter a material with a much higher dielectric constant, like liquid water, the impedance mismatch causes a strong reflection. On Mars, the MARSIS instrument sends low-frequency radar pulses that can travel through kilometers of ice. When they hit the base of the ice cap, if there's liquid water, the reflection is significantly brighter than from rock. This bright reflector, along with its shape and the radar characteristics, indicates a subglacial lake. The pressure from the ice above and possible salts can keep water liquid at temperatures below freezing. This method is powerful because it allows us to explore beneath the surface without landing, and it has implications for finding potentially habitable environments on other planets.